Rhododendron plant named ‘UMNAZ633’

ABSTRACT

A new cultivar of hybrid Rhododendron plant named ‘UMNAZ633’ that is characterized by its rounded plant form with compactly branched plant habit; reaching 1 m in height and width after 15 years of growth, its abundance of flowers that are red in color in mid to late May in Excelsior, Minn. with the red color retained throughout the bloom period without substantial fading prior to abscission, its good cold hardiness with flower buds that are resistant to cold damage and consistently bloom even when exposed to temperatures as low as −25°, and its foliage with relatively good tolerance to powdery mildew.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under MN Agricultural Experiment Station (USDA/Hatch) MN 21-078, MN 21-058, and MN 22-055. The government has certain rights in the invention.

Botanical classification: Rhododendron hybrida.

Cultivar designation: ‘UMNAZ633’.

BACKGROUND OF THE INVENTION

The present invention relates to a new and distinct cultivar of Rhododendron plant of hybrid origin, botanically known as Rhododendron hybrida ‘UMNAZ633’ and will be referred to hereafter by its cultivar name, ‘UMNAZ633’. ‘UMNAZ633’ is a new cultivar of deciduous Rhododendron grown for use as a landscape plant.

The new cultivar was developed through an on-going breeding program conducted by the Inventors in Excelsior, Minn. The objectives of the breeding program are to develop new cultivars of Rhododendron that exhibit cold tolerance, excellent floral displays, disease-resistant foliage, and compact plant habits.

The new cultivar was derived from a cross made by the Inventors on May 25, 2000 between Rhododendron hybrida ‘UMNAZ393’ (not patented), as the female parent, and Rhododendron hybrida ‘UMNAZ602’ (not patented), as the male parent. The Inventors selected ‘UMNAZ633’ as a single unique plant amongst the seedlings that resulted from the above cross in June of 2009.

Asexual propagation of the new cultivar was first accomplished by softwood stem cuttings in Excelsior, Minn., in June of 2009 by one of the Inventors. Asexual propagation by softwood stem cuttings and tissue culture has determined that the characteristics of the new cultivar are stable and are reproduced true to type in successive generations.

SUMMARY OF THE INVENTION

The following traits have been repeatedly observed and represent the characteristics ‘UMNAZ633’. These attributes in combination distinguish ‘UMNAZ633’ as a new and distinct cultivar of Rhododendron.

-   -   1. ‘UMNAZ633’ exhibits a rounded plant form with compactly         branched plant habit; reaching 1 m in height and width after 15         years of growth.     -   2. ‘UMNAZ633’ exhibits an abundance of flowers that are red in         color in mid to late May in Excelsior, Minn. with the red color         retained throughout the bloom period without substantial fading         prior to abscission.     -   3. ‘UMNAZ633’ exhibits good cold hardiness with flower buds that         are resistant to cold damage and consistently bloom even when         exposed to temperatures as low as −25° F.     -   4. ‘UMNAZ633’ exhibits foliage with relatively good tolerance to         powdery mildew.

The female parent of ‘UMNAZ633’, ‘UMNAZ393’, differs from ‘UMNAZ633’ in having a less compact plant habit with more open branching, flowers that fade to a pale red prior to abscission, and fewer flower buds. The male parent, ‘UMNAZ602’, differs from ‘UMNAZ633’ in having flowers that are orange-red in color with an orange-gold patch on the top petal, flower buds that are less cold tolerant, and more susceptibility to powdery mildew and leaf abscission. ‘UMNAZ633’ can also be most closely compared to the Rhododendron cultivars ‘Arneson Flame’ (not patented) and ‘UMNAZ502’. ‘Arneson Flame’ is similar to ‘UMNAZ633’ in having flowers that are red in color and a deciduous growth habit. ‘Arneson Flame’ differs from ‘UMNAZ633’ in having flower buds and stems that are less cold tolerant in U.S.D.A. Zone 4 winters. ‘UMNAZ502’ is similar to ‘UMNAZ633’ in having flowers that are red in color and flower buds that are capable of surviving temperatures of −25° F. ‘UMNAZ502’ differs from ‘UMNAZ633’ in having smaller flowers, flower buds that exhibit less red pigmentation, a larger plant size, and a less compact and less densely branched plant habit.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying colored photographs were taken of a 17-year-old plant of the new cultivar as grown outdoors in an evaluation nursery in Excelsior, Minn.

The photograph in FIG. 1 provides a view of the overall plant habit of ‘UMNAZ633’ in bloom.

The photograph in FIG. 2 provides a close-up view of a flower and the spring foliage of ‘UMNAZ633’.

The photograph in FIG. 3 provides a close-up view of the flower buds in fall with a comparison of the flower buds of ‘UMNAZ502’ on the left and ‘UMNAZ633’ on the right.

The colors in the photographs are as close as possible with the digital photography and printing techniques utilized and the color codes in the detailed botanical description accurately describe the new Rhododendron.

DETAILED BOTANICAL DESCRIPTION OF THE PLANT

The following is a detailed description of 8-year-old plants of the new cultivar as grown outdoors in an evaluation nursery in Excelsior, Minn. The phenotype of the new cultivar may vary with variations in environmental, climatic, and cultural conditions, as it has not been tested under all possible environmental conditions. The color determination is in accordance with The 2015 Colour Chart of The Royal Horticultural Society, London, England, except where general color terms of ordinary dictionary significance are used.

-   General characteristics:     -   -   Blooming period.—7 to 11 days, typically commencing at the             end of April in Excelsior, Minn.         -   Plant type.—Deciduous shrub.         -   Plant habit.—Rounded and densely branched.         -   Height and spread.—Reaches 1 m in height and width after 15             years in the landscape.         -   Cold hardiness.—Plant is hardy to at least U.S.D.A. Zone 4a,             flower buds are resistant to cold damage when exposed to             temperatures down to −25 F.         -   Diseases and pests.—Observed to have foliage that is             moderately tolerant to powdery mildew (caused by             Microsphaera azalea), no susceptibility or resistance to             other diseases or insect pests has been observed.         -   Root description.—Fibrous and 165A in color.         -   Propagation.—Tissue culture (preferred) or softwood stem             cuttings.         -   Growth rate.—Slow to moderate.         -   Root development.—4 to 8 weeks for root initiation and 2             years to produce a young plant from a rooted cutting. -   Stem description:     -   -   Shape.—Round.         -   Stem color.—Young growth; 145A and 145B transitioning to             N167A to 165B during growing season, mature wood; a blend of             156C and 202C.         -   Stem size.—Main stems; an average of 36 cm in length and 1             cm in width, lateral branches; an average of 17 cm in length             and 5 mm in width, current year's growth; an average of 9 cm             in length and 3 mm in width.         -   Stem surface.—Young growth; slightly rough to the touch,             moderately covered in woolly pubescent hairs, up to 2 mm in             length and 155A in color to matching stem surface, becoming             smooth and glabrous in summer, mature wood; somewhat rough,             dull and glabrous.         -   Stem aspect.—Held mainly vertical to slightly outward.         -   Stem strength.—Strong.         -   Branching.—Densely branched. -   Foliage description:     -   -   Leaf shape.—Elliptic to obovate.         -   Leaf division.—Simple.         -   Leaf base.—Cuneate.         -   Leaf apex.—Acute to obtuse.         -   Leaf venation.—Pinnate, upper surface midrib 146C in color             with lateral vein color matching leaf coloration, lower             surface midrib and lateral veins 162A and 151A in color with             short ciliate hairs on midrib that match surface color.         -   Leaf margins.—Entire and fringed with hairs; 0.5 mm in             length and 155D in color.         -   Leaf attachment.—Petiolate.         -   Leaf arrangement.—Alternate, leaf clusters whorled.         -   Leaf internode length.—Ranges from 0.2 to 3.2 cm becoming             progressively smaller from base to apex of shoot, 0.2 to 0.4             cm in whirl of foliage at shoot apex.         -   Leaf orientation.—Held mostly upright, some slightly             downward.         -   Leaf aspect.—Whole leaf cupped upward with apex cupping             downward.         -   Leaf surface.—Upper surface; smooth, glabrous, shiny with a             thick texture, lower surface; smooth, dull, glabrous except             prominent midrib with short cilia; <0.5 mm in length and             155B in color.         -   Leaf color.—Young leaves upper surface; 144A with transient             53A overtone in spring young leaves lower surface; a blend             of 144A and 146B, mature leaves upper surface; a blend 146A             and 146B, mature leaves lower surface; 147C, fall color             upper surface; a blend of 46A, 147A and 166A, fall color             lower surface; ranges from 144D to 183B.         -   Leaf size.—Ranging from 2.0 to 7.0 cm in length and 1.4 to             3.5 cm in width.         -   Leaf quantity.—An average of 10 leaves per lateral branch 12             cm length.         -   Petioles.—Average of 5 mm in length and 1 to 2 mm in             diameter, 143B and 144A in color, smooth and lightly             pubescent surface. -   Flower description:     -   -   Inflorescence type.—Umbellate raceme.         -   Lastingness of flowers.—Individual flowers last 2 to 4 days,             total duration of bloom ranges from 7 to 11 days,             self-cleaning.         -   Inflorescence size.—Average of 10 cm in height and 12 cm in             width.         -   Flower size.—An average of 4.5 cm in depth and 5.5 cm in             diameter.         -   Flower fragrance.—Lightly fragrant.         -   Flower shape.—Broad, tubular funnel.         -   Flower number.—Average of 7 per inflorescence.         -   Flower aspect.—Outward.         -   Flower bud.—1 to 4 per terminal shoot whorl, obovate in             shape, round in cross- section, pointed apex, an average of             1 cm in length and 0.6 cm in diameter, color; initially a             blend of 142A and 143C in late-summer, developing 53B near             the apex and margins of bud scales, especially on             sun-exposed side of buds in the fall, winter buds; 166B in             color with regions of 175A and 175B and some buds exhibiting             150B at base.         -   Flower attachment.—Pedicellate.         -   Petal number.—5.         -   Petal shape.—Ovate to elliptic.         -   Petal color.—Expanding flowers buds (balloon stage) are 60A             in color and covered with fine hairs; 0.5 mm in length and             155D in color on lower half of the expanding corolla, upper             surface color when fully open 44A and 45A, lower surface             when fully open 45A and 45B often with a darker (46A)             central axis.         -   Petal surface.—Upper surface; glabrous and smooth to             slightly rugose, lower surface; smooth, glabrous except for             moderate pubescence along the midline.         -   Petal margins.—Entire, undulate.         -   Petal apex.—Acuminate to obtuse, often recurved.         -   Petal base.—Fused.         -   Petal size.—An average of 2.7 cm in length and 1.7 cm in             width.         -   Sepal number.—5.         -   Sepal shape.—Oblong to lanceolate.         -   Sepal margin.—Entire and fringed with hairs; 0.5 mm in             length and 155A in color.         -   Sepal size.—Average of 3.0 mm in length and 2.5 mm in width.         -   Sepal surface.—Pubescent on upper and lower surface.         -   Sepal apex.—Obtuse to rounded.         -   Sepal base.—Fused.         -   Sepal color.—Immature upper surface; (not visible) enclosing             base of the corolla tube, immature lower surface; 145A,             mature upper surface; 144B and 58A near apex, mature lower             surface a blend of 144A and 144B.         -   Calyx.—Shallow cup shape an average of 4.0 mm in length and             6.0 mm in diameter, with individual sepals separated by             rounded sinuses.         -   Pedicels.—An average of 1 cm in length and 1.5 mm in             diameter, smooth and lightly pubescent surface, 146C in             color. -   Reproductive organs:     -   -   Gynoecium.—1 Pistil, stigmas; elliptic to round in shape,             147A in color, 2 mm in diameter, style; average 7.05 cm in             length, curved and 46A in color, ovary; 146C in color.         -   Androecium.—Stamens typically 5 anthers; oblong to             elliptical in shape, 2 mm long and 1.5 mm wide, 164A to 164C             in color; pollen 155D in color, filament; average 6.0 cm in             length, curved and 44B in color.         -   Fruit/seeds.—Dehiscent, 5-valved capsule with an average of             2 cm length and 7.5 mm in width, 145B in color in late             summer transitioning to 166A exterior color and 167C to 167D             interior color at maturity, seed; an average of 3 mm in             length and 1.5 mm in width, 164A and 166C in color, 50 to             150 seeds per capsule. 

It is claimed:
 1. A new and distinct cultivar of Rhododendron plant named ‘UMNAZ633’ as herein illustrated and described. 